Food Waste Digester

Purdue University has collaborated with the city of West Lafayette on its state-of-the-art Wastewater Treatment Plant. The onsite digester equipment allows the city to convert fats, oils, and grease (called FOG) and food scrap waste from Purdue University to produce energy used to augment the plant's electricity usage.

This innovative waste-to-energy treatment system reduces the plant's overall operating and maintenance costs, has decreased the overall carbon footprint of the community, and reduces waste going to landfills.

During the academic semester, a little over 2000 pounds per week of pre- and post-consumer food scraps from Purdue University’s residence halls and dining courts are fed into the food digester. The system generates as much as 18% of the total energy required to operate the Wastewater Treatment Plant.

In 2010, the Treatment Plant was upgraded to replace two 50-year-old anaerobic digesters with a new, efficient, externally-pumped mixing system housed inside a new digester building. The building also incorporates a new sludge heating system, with heat exchangers and a boiler, and a co-generation system that uses digester gas (methane) to generate electricity and heat. The heat is used to raise the temperature of the contents of the digesters in order for the microorganisms to break down the waste. The old digesters had a boiler that used a lot of natural gas, while this new system relies on the heat generated by the microturbines, and only has a natural gas boiler for backup purposes. This arrangement results in substantial financial savings for the city.

The university was already separating out the food waste from other waste for a composting project. This arrangement continued even after the composting project ended, and it was a perfect opportunity to take this preprocessed pulp matter and use it in the digesters to increase energy production, with only minimal added expense initially. Because the university is close to the treatment plant, transporting the food waste there costs far less hauling it to the landfill.

The normal digestion process produced 37 cu. ft. of gas per minute. Adding FOG doubled the amount of digester gas. The city of West Lafayette is hoping to eventually expand the food waste program to commercial facilities such as the local Wal-Mart and food markets.